Pilot makes some pretty nice stuff under $100, but for the money, I'd still recommend a Lamy Safari or AlStar if you don't already have one. The Pilot Metropolitan is another good option for the money.
For just a little over $100, look at the MonteVerde Invincia Deluxe. I have the carbon fiber / black one and really enjoy it. Nib options from extra fine through calligraphy broad. The standard Invincia is less than $100, but doesn't use as nice of materials.
If you are OK doing KickStarter, there's a really slick-looking Ti or Al pen being backed at the moment, called the Stilform Ink. The campaign is over now, but you can still pre-order one. Delivery will likely be early next summer. Definitely the best pen option under $100 right now if you ask me. They're using Peter Bock nibs, you get a silver steel one included for the price of the pen, or you can upgrade to several other nib options. I have multiple coming, in both metals.
If you haven't already, look at Goulet pens. Tons to choose from on their site, awesome reviews and helpful tips, and they have excellent customer service if you ever need it.